Index

Symbols  A  B  C  D  E  F  G  H  I  K  L  M  N  O  P  R  S  T  U  V

 

Symbols

#, comments in configuration statements    1
( ), in syntax descriptions    1
*,G notation, for multicast forwarding states    1
< >, in syntax descriptions    1
[ ], in configuration statements    1
{ }, in configuration statements    1
| (pipe), in syntax descriptions    1
 

A

ABRs     See area border routers    
access control lists (ACLs)     See stateless firewall filters    
ACLs     See stateless firewall filters    
action modifiers, stateless firewall filters    
list of    12
actions    
default, routing policy    1
final, routing policy    1
modifiers, list of    12
route list match types    1
routing policy    1
routing policy, summary of    1
stateless firewall filters, list of    12
active routes, versus passive routes    1
address match conditions    12
addresses    1,  See also IPv4 addressing; IPv6 addressing    
IS-IS NETs    1,  See also NETs    
IS-IS NSAP addresses    1
multicast ranges    1
adjacencies, IS-IS    
hello PDUs    1,  See also IS-IS    
administrative scoping    1
advertisements     See LSAs; route advertisements    
aggregation, route    1
always compare, BGP MED option    1
area border routers    
backbone area     See backbone area    
description    1
areas     See area border routers; backbone area; IS-IS, areas; NSSAs; stub areas    
AS path    
description    1
forcing by MED    1
prepending    1
role in BGP route selection    1
ASs (autonomous systems)    
area border routers    1
breaking into confederations    1
description    1
IS-IS networks    1
stub areas     See stub areas    
authentication    
OSPF, MD5    1
OSPF, plain-text passwords    1
RIPv2, MD5    1
RIPv2, plain-text passwords    1
Auto-RP    1
autonomous systems     See ASs    
 

B

backbone area    
configuring    1
description    1
BGP (Border Gateway Protocol)    
AS path    1,  See also AS path    
confederations     See BGP confederations    
external    1,  See also EBGP    
full mesh requirement    1
injecting OSPF routes into BGP    1
internal    1,  See also IBGP    
local preference    1
MED metric    1,  See also MED    
origin value    1
overview    12
peering sessions     See BGP peers; BGP sessions    
point-to-point internal peer session (configuration editor)    1
point-to-point peer session (configuration editor)    1
policy to make routes less preferable    1
requirements    1
route reflectors     See BGP route reflectors    
route selection process    1,  See also route selection    
route-flap damping    1
scaling techniques    1
session establishment    1
session maintenance    1
verifying BGP configuration    1
verifying BGP groups    1
verifying BGP peers (neighbors)    1
verifying peer reachability    1
BGP confederations    
creating (configuration editor)    1
description    1
route-flap damping    1
BGP groups    
confederations (configuration editor)    1
verifying    1
BGP messages    
to establish sessions    1
update, to maintain sessions    1
BGP neighbors     See BGP peers    
BGP peers    
directing traffic by local preference    1
external (configuration editor)    1
internal (configuration editor)    1
monitor probes    1
peer address, role in route selection    1
point-to-point connections    1
sessions between peers    1
verifying    12
verifying reachability    1
BGP route reflectors    
cluster of clusters    1
clusters, role in route selection    1
creating (configuration editor)    1
description    1
multiple clusters    1
BGP sessions    
configured at both ends    1
establishment    1
maintenance    1
point-to-point external (configuration editor)    1
point-to-point internal (configuration editor)    1
sample peering session    1
bit-field logical operators, stateless firewall filters    1
bit-field match conditions    1
bit-field synonym match conditions    1
bootstrap router    1
Border Gateway Protocol     See BGP    
braces, in configuration statements    1
brackets    
angle, in syntax descriptions    1
square, in configuration statements    1
branches    1,  See also multicast    
BSR (bootstrap router)    1
 

C

chained stateless firewall filters    1
Cisco non-deterministic, BGP MED option    1
clusters     See BGP route reflectors    
comments, in configuration statements    1
complete sequence number PDU (CSNP)    1
confederations     See BGP confederations    
connectivity    
bidirectional (BGP)    1
bidirectional (OSPF)    1
unidirectional (RIP)    1
conventions    
notice icons    1
text and syntax    1
cost, of a network path     See path cost metrics    
CSNP (complete sequence number PDU)    1
curly braces, in configuration statements    1
customer support    1
contacting JTAC    1
 

D

defaults    
routing policy actions    1
denial-of-service attacks, preventing    1
dense routing mode, caution for use    1,  See also multicast routing modes    
designated router, OSPF    
controlling election    1
description    1
designated router, stopping outgoing PIM register messages on    1
diagnosis    
displaying stateless firewall filter configurations    123
displaying stateless firewall filter statistics    1
displaying static routes in the routing table    1
verifying BGP configuration    1
verifying BGP groups    1
verifying BGP peer reachability    1
verifying BGP peers (neighbors)    1
verifying firewall filter handles fragments    1
verifying multicast IGMP versions    1
verifying multicast SAP and SDP configuration    1
verifying OSPF host reachability    1
verifying OSPF neighbors    1
verifying OSPF routes    1
verifying OSPF-enabled interfaces    1
verifying PIM mode and interface configuration    1
verifying PIM RPF routing table    1
verifying PIM RPs    1
verifying RIP host reachability    1
verifying RIP message exchange    1
verifying RIP-enabled interfaces    1
verifying stateless firewall filter actions    1
verifying stateless firewall filter DoS protection    1
verifying stateless firewall filter flood protection    1
verifying stateless firewall filters with packet logs    1
discard, filter action    
automatic, stateless firewall filters    1
Distance Vector Multicast Routing Protocol    1
distance-vector routing protocols    1,  See also RIP    
documentation    
comments on    1
DoS (denial-of-service) attacks, preventing    1
downstream interfaces    1,  See also multicast    
DR     See designated router    
DVMRP (Distance Vector Multicast Routing Protocol)    1
dynamic routing    1
 

E

EBGP (external BGP)    
description    1
route-flap damping    1
EGPs (exterior gateway protocols)    1
exact route list match type    1
export statement, for routing policies    1
exterior gateway protocols    1
external BGP     See EBGP    
external paths, role in BGP route selection    1
 

F

firewall filters    
stateless firewall filters    1,  See also stateless firewall filters    
term number caution    1
verifying fragment handling    1
flap damping    1
parameters    1
flooding, preventing    1
flow control, actions in routing policies    1
font conventions    1
forwarding classes    
policy to group source and destination prefixes    1
forwarding states, multicast notation    1
forwarding table    
controlling OSPF routes in    1
controlling static routes in    12
description    1
MED to determine routes in    1
from statement, routing policy match conditions    1
full mesh requirement    
description    1
fulfilling with confederations    1
fulfilling with route reflectors    1
 

G

ge-0/0/0, disabling PIM on    1
groups    
BGP     See BGP groups    
OSPF areas    1
RIP routers    1
 

H

handling packet fragments    1
hardware    
supported platforms    1
hello PDUs    1
hold time, to maintain a session    1
hop count, maximizing    1,  See also RIP    
host reachability    
verifying for an OSPF network    1
verifying for RIP network hosts    1
hostname    
IS-IS identifier-to-hostname mapping    1
 

I

IBGP (internal BGP)    
description    1
full mesh (configuration editor)    1
ICMP (Internet Control Message Protocol), policers    1
IGMP (Internet Group Management Protocol)    
IGMPv1    1
IGMPv2    1
IGMPv3    1
setting the version    1
verifying the version    1
IGP plus MED, BGP option    1
IGP route metric, role in BGP route selection    1
IGPs (interior gateway protocols)    1
import statement, for routing policies    1
incoming metric (RIP)    
description    1
inet routing table    1
interior gateway protocols    1
internal BGP     See IBGP    
Internet Control Message Protocol policers    1
Internet Group Management Protocol     See IGMP    
Internet routing, with BGP    1
IP addresses    
as IS-IS system identifiers    1
IS-IS (Intermediate System-to-Intermediate System)    
adjacency establishment with hello PDUs    1
areas    1
ASs    1
CSNPs    1
hello PDUs    1
LSPs    1
NETs    1,  See also NETs    
NSAP addresses    1
overview    1
path selection    1
PSNPs    1
system identifiers    1,  See also system identifiers    
ISO network addresses, for IS-IS routers    1
 

K

keepalive messages, for session hold time    1
 

L

leaves    1,  See also multicast    
Level 1 areas, IS-IS    1
Level 2 areas, IS-IS    1
link-state advertisements     See LSAs    
link-state PDUs     See LSPs    
local preference    
description    1
high value preferred    1
role in BGP route selection    1
longer route list match type    1
loopback interface, applying stateless firewall filters to (configuration editor)    1
LSAs (link-state advertisements)    
description    1
three-way handshake    1
LSPs (link-state PDUs)    
CSNPs    1
overview    1
PSNPs    1
 

M

MAC (media access control) addresses    
as IS-IS system identifiers    1
management interfaces, disabling PIM on    1
manuals    
comments on    1
match conditions    
routing policy    1
routing policy, summary of    1
stateless firewall filters    1
stateless firewall filters, summary    12
match types    1
maximum hop count, RIP    1
MED (multiple exit discriminator)    
always compare option    1
Cisco non-deterministic option    1
default use    1
description    1
path selection options    1
plus IGP option    1
role in BGP route selection    1
metrics     See MED; path cost metrics    
MSDP (Multicast Source Discovery Protocol)    1
multiarea network, OSPF    1
multicast    
*,G notation    1
administrative scoping    1
architecture    1
Auto-RP    1
BSR    1
downstream interface    1
DVMRP    1
forwarding state notation    1
IGMP     See IGMP    
IP address ranges    1
MSDP    1
network elements    1
overview    1
PGM    1
PIM dense mode     See PIM    
PIM register messages     See PIM register messages    
PIM source-specific multicast (SSM)    1
PIM sparse mode     See PIM    
preparation    1
preventing routing loops    1
protocols    1
reverse-path forwarding (RPF)    1
routing modes     See multicast routing modes    
S,G notation    1
SAP and SDP     See SAP; SDP    
session announcements    1
shortest-path tree (SPT)    1
static RP    1,  See also RP    
subnetwork leaves and branches    1
upstream interface    1
verifying IGMP versions    1
verifying PIM mode and interface configuration    1
verifying PIM RPF routing table    1
verifying PIM RPs    1
verifying SAP and SDP configuration    1
multicast routing modes    
dense mode    1
dense mode, caution for use    1
sparse mode    1
Multicast Source Discovery Protocol    1
multiple exit discriminator     See MED    
 

N

n-selectors, in IS-IS NET addresses    1
neighbors     See adjacencies, IS-IS; BGP peers; OSPF neighbors; RIP neighbors    
NETs (network entity titles)    
n-selectors    1
parts    1
system identifier    1
network entity titles     See NETs    
network interfaces    
enabling PIM on    1
multicast, upstream and downstream    1
verifying PIM on    1
verifying RIP message exchange    1
verifying RIP on    1
network service access point (NSAP) addresses for IS-IS routers    1
networks    
description    1
designated router     See designated router, OSPF    
path cost metrics     See path cost metrics    
sample BGP AS path    1
sample BGP confederations    1
sample BGP local preference use    1
sample BGP MED use    1
sample BGP peer session    1
sample BGP route reflector (one cluster)    1
sample BGP route reflectors (cluster of clusters)    1
sample BGP route reflectors (multiple clusters)    1
sample distance-vector routing    1
sample multiarea OSPF routing    1
sample OSPF backbone area    1
sample OSPF network with stubs and NSSAs    1
sample OSPF topology    12
sample poison reverse routing    1
sample route advertisement    1
sample route aggregation    1
sample routing topology    1
sample split horizon routing    1
sample unidirectional routing    1
static routing    1
next hop    
qualified, for static routes    1
role in BGP route selection    1
not-so-stubby areas     See NSSAs    
notice icons    1
NSAP (network service access point) addresses for IS-IS routers    1
NSSAs (not-so-stubby areas)    
creating (configuration editor)    1
description    1
example    1
numeric range match conditions    12
 

O

Open Shortest Path First protocol     See OSPF    
origin, of BGP route    1
orlonger route list match type    1
OSPF (Open Shortest Path First)    
area border routers     See area border routers    
areas    1,  See also area border routers; backbone area; NSSAs; stub areas    
authenticating exchanges (OSPFv2 only)    1
backbone area     See backbone area    
controlling designated router election    1
controlling route cost    1
designated router     See designated router, OSPF    
enabling, description    1234
ensuring efficient operation    1
injecting OSPF routes into BGP    1
LSAs    1
multiarea network (configuration editor)    1
NSSAs     See NSSAs    
overview    12
path cost metrics     See path cost metrics    
requirements    1
route preferences    1
router ID (configuration editor)    1
sample network topology    12
single-area network (configuration editor)    1
stub areas     See stub areas    
three-way handshake    1
tuning an OSPF network    1
verifying host reachability    1
verifying neighbors    1
verifying RIP-enabled interfaces    1
verifying routes    1
OSPF interfaces    
verifying    1
OSPF neighbors, verifying    1
outgoing metric (RIP)    
description    1
 

P

packet-mode    
action modifier    12
packets    
handling packet fragments    1
handling packet fragments (configuration editor)    1
RIP, description    1
parentheses, in syntax descriptions    1
partial sequence number PDU (PSNP)    1
passive routes, rejection, in static routing    1
password    
for OSPFv2 authentication    1
for RIPv2 authentication    1
path cost metrics    
for BGP, description    1,  See also MED    
for OSPF routes, description    1
for OSPF routes, modifying    1
for RIP routes, description    1
for RIP routes, modifying    12
path selection, IS-IS    1
path-vector protocol     See BGP    
PDUs (protocol data units)    
CSNPs    1
hello PDUs    1
LSPs    1
overview    1
PSNPs    1
peering sessions     See BGP peers; BGP sessions    
permanent routes, adding    1
PGM (Pragmatic General Multicast)    1
PIM (Protocol Independent Multicast)    
dense mode    1
disabling on the network management interface    1
register messages     See PIM register messages    
RPF routing table group    1
source-specific multicast (SSM)    1
sparse mode    1
static RP router    1
supported versions    1
verifying the mode    1
verifying the RP    1
PIM register messages    
filtering    1
incoming, rejecting on an RP    1
outgoing, rejecting on a designated router    1
reject policy on designated router    1
reject policy on RP router    1
ping command (stateless firewall filter)    1
explanation    1
Ping Host page, output for BGP    12
poison reverse technique    1
policers    
for stateless firewall filters    1
policy     See routing policies    
Pragmatic General Multicast    1
preferences    
for OSPF routes    1
for static routes    1
prefix-length-range match type    1
propagation, suppressing    1
protocol data units     See PDUs    
Protocol Independent Multicast     See PIM    
protocols    
Auto-RP    1
BGP     See BGP    
distance vector     See RIP    
DVMRP    1
EGPs    1
IGMP     See IGMP    
IGPs    1
IS-IS     See IS-IS    
MSDP    1
multicast     See multicast    
OSPF     See OSPF    
overview    1
path vector     See BGP    
PGM    1
PIM dense mode     See PIM    
PIM source-specific multicast (SSM)    1
PIM sparse mode     See PIM    
RIP     See RIP    
SAP and SDP     See SAP; SDP    
PSNP (partial sequence number PDU)    1
 

R

reachability    
verifying for a RIP network    1
verifying for BGP peers    1
verifying for OSPF network hosts    1
real-time performance monitoring (RPM), for BGP peers    1
rejecting    
unauthorized PIM registration    1
reverse-path forwarding     See RPF    
RIB     See routing table    
RIP (Routing Information Protocol)    
authentication (RIPv2 only)    1
authentication (RIPv2 only), configuring    12
basic network (configuration editor)    1
distance vector protocol    1
efficiency techniques    1
maximum hop count    1
overview    12
packets    1
path cost metrics     See path cost metrics    
poison reverse technique    1
requirements    1
routing policy (configuration editor)    1
split horizon technique    1
traffic control with metrics     See path cost metrics    
traffic control with metrics, configuring    12
unidirectional limitations    1
verifying host reachability    1
verifying RIP message exchange    1
verifying RIP-enabled interfaces    1
RIP neighbors, verifying    1
RIPng (Routing Information Protocol next generation)    
overview    1
route advertisements    
AS path in    1
BGP, update messages    1
description    1
external, EBGP    1
internal, IBGP    1
LSAs    1
stub areas and NSSAs, to control    1
route aggregation    1
route injection    1
route list match types    1
route manipulation actions, routing policies    1
route origin, role in BGP route selection    1
route redistribution    1
route reflectors     See BGP route reflectors    
route selection    
BGP process for    1
BGP, determining by AS path    1
BGP, determining by local preference    1
BGP, determining by MED metric    1
BGP, lowest origin value preferred    1
static routes, defining    12
route-flap damping    1
parameters    1
router ID, role in BGP route selection    1
routing    1,  See also protocols; routing policies; routing solutions    
advertisements    1
aggregation    1
BGP     See BGP    
dynamic    1
filtering routes with policies    1
filtering traffic through a stateless firewall    1
forwarding tables    1
from one source to many destinations    1
in multiple ASs with BGP    1
in one AS with OSPF    1
in one AS with RIP    1
IS-IS     See IS-IS    
multicast     See multicast    
neighbors     See BGP peers; OSPF neighbors; RIP neighbors    
OSPF     See OSPF    
protocol overview    1
RIP     See RIP    
RIP statistics    1
RIPng     See RIPng    
routing tables    1
static     See static routing    
Routing Engine    
handling packet fragments for (configuration editor)    1
protecting against DoS attacks    1
protecting against untrusted services and protocols (configuration editor)    1
routing information base     See routing table    
Routing Information Protocol     See RIP    
routing policies    
actions    1
applying    1
configuration tasks    123456
default actions    1
export statement    1
final actions    1
forwarding class with source and destination    1
grouping source and destination prefixes    1
import statement    1
injecting routes from one protocol into another    1
making BGP routes less preferable    1
match conditions    1
overview    1
PIM register messages     See PIM register messages    
policy name    1
preparation    1
prepending AS paths    1
reducing update messages with flap damping    1
RIP routing policy (configuration editor)    1
route redistribution    1
route-flap damping    1
terms    1
terms, creating    1
routing solutions    
BGP confederations, for scaling problems    1
BGP route reflectors, for scaling problems    1
BGP scaling techniques    1
controlling designated router election    1
controlling OSPF route cost    1
controlling OSPF route selection    1
controlling RIP traffic with the incoming metric    1
controlling RIP traffic with the outgoing metric    1
designated router, to reduce flooding    1
directing BGP traffic by local preference    1
filtering unwanted services and protocols    1
handling packet fragments    1
handling packet fragments (configuration editor)    1
making BGP routes less preferable    1
multicast administrative scoping    1
multicast reverse-path forwarding (RPF)    1
multicast shortest-path tree (SPT)    1
NSSAs, to control route advertisement    1
path cost metrics, for packet flow control     See path cost metrics    
poison reverse, for traffic reduction    1
preventing multicast routing loops    1
protecting against DoS attacks    1
reducing update messages with flap damping    1
routing policies    1
securing OSPF routing (OSPFv2 only)    1
split horizon, for traffic reduction    1
stateless firewall filters    1
static route control techniques    1
stub areas, to control route advertisement    1
routing table    
controlling static routes in    12
description    1
displaying static routes in    1
RPF group, for multicast    1
sample distance-vector routing    1
updates, limitations in RIP    1
verifying for RPF    1
verifying OSPF routes    1
RP (rendezvous point)    
PIM register messages, incoming, rejecting    1
PIM register messages, outgoing, stopping    1
reject policy for incoming PIM register messages    1
same reject policy on RP routers in a network    1
static    1
verifying    1
RP router     See RP    
RPF (reverse-path forwarding)    
description    1
routing table group    1
verifying the routing table    1
RPM, for BGP peers    1
 

S

S,G notation, for multicast forwarding states    1
sample configurations    
firewall filter configurations    123
SAP (Session Announcement Protocol)    
description    1
session announcements    1
verifying    1
scaling BGP     See BGP confederations; BGP route reflectors    
scoping, administrative    1
SDP (Session Discovery Protocol)    
description    1
session announcements    1
verifying    1
security    
MD5 authentication for OSPF    1
MD5 authentication for RIPv2    1
password authentication for OSPFv2    1
password authentication for RIPv2    1
stateless firewall filters    1
Session Announcement Protocol     See SAP; SDP    
sessions    
announcements, multicast    1
BGP session establishment    1
BGP session maintenance    1
shortest path first algorithm    1
shortest-path tree    1
show bgp group command    1
show bgp neighbor command    12
explanation    12
show bgp summary command    12
explanation    12
show firewall command    123
show firewall filter protect-RE command    1
show firewall log command    1
show igmp interface command    1
explanation    1
show interfaces lo0 command    1
show isis adjacency brief command    1
show isis adjacency extensive command    1
explanation    1
show multicast rpf command    1
explanation    1
show ospf interface command    1
explanation    1
show ospf neighbor command    1
show ospf route command    12
explanation    12
show pim interface command    1
explanation    1
show pim rps command    1
explanation    1
show rip neighbor command    1
show rip statistics command    1
show route summary command    12
explanation    1
show route terse command    1
show sap listen command    1
explanation    1
show isis interface brief command    1
show isis interface detail command    1
explanation    1
single-area network, OSPF    1
source-specific multicast    1
sparse mode     See multicast routing modes    
SPF (shortest path first) algorithm    1
split horizon technique    1
SPT (shortest-path tree)    1
ssh command    1
stateless firewall filters    
actions and action modifiers    12
applying to an interface (configuration editor)    1
automatic discard rule    12
bit-field logical operators    1
chained multiple filters    1
displaying configurations    123
displaying statistics    1
handling packet fragments    1
handling packet fragments (configuration editor)    1
match conditions    1
multiple filters, chained    1
overview    1
planning    123
policers for    1
preparation    1
protecting the Routing Engine against TCP floods    1
protecting the Routing Engine against untrusted protocols (configuration editor)    1
protecting the Routing Engine against untrusted services (configuration editor)    1
sample terms, to filter fragments    1
sample terms, to filter services and protocols    1
sequences    1
terms, overview    1
typical, planning    1
verifying actions    1
verifying configuration    123
verifying flood protection    1
verifying packet logging    1
static routes    
configuring basic routes (configuration editor)    1
controlling    1
controlling in routing and forwarding tables    1
default properties    1
default properties, setting    1
defining route selection    12
preferences    1
preventing readvertisement    1
qualified next hops    1
rejecting passive traffic    1
requirements    1
route retention    1
verifying    1
static routing    
description    1
overview    1,  See also static routes    
static RP router    1,  See also RP    
statistics    
RIP    1
stateless firewall filters    1
stub areas    
creating (configuration editor)    1
description    1
example    1
sub-ASs, BGP    1
subautonomous systems, BGP    1
subnetworks    
description    1
route aggregation    1
subnetworks, multicast leaves and branches    1
support, technical     See technical support    
syntax conventions    1
system identifier, IS-IS    
all zeros not supported    1
formats, MAC or IP address    1
identifier-to-hostname mapping    1
overview    1
 

T

TCP policers    1
technical support    
contacting JTAC    1
telnet command    1
terms    
in a routing policy    1
in a routing policy, creating    1
stateless firewall filters, overview    1
three-way handshake    1
through route list match type    1
to statement, routing policy match conditions    1
topology    
sample BGP AS path    1
sample BGP confederations    1
sample BGP local preference use    1
sample BGP MED use    1
sample BGP peer session    1
sample BGP route reflector (one cluster)    1
sample BGP route reflectors (cluster of clusters)    1
sample BGP route reflectors (multiple clusters)    1
sample distance-vector routing    1
sample multiarea OSPF routing    1
sample OSPF backbone area    1
sample OSPF network    12
sample OSPF network with stubs and NSSAs    1
sample poison reverse routing    1
sample route advertisement    1
sample route aggregation    1
sample router network    1
sample split horizon routing    1
sample static route    1
sample unidirectional routing    1
topology database, OSPF    1
Traceroute page    
results for OSPF    1
results for RIP    1
traffic    
controlling with incoming RIP metric    1
controlling with outgoing RIP metric    1
filtering through a stateless firewall    1
 

U

upstream interfaces    1,  See also multicast    
upto route list match type    1
 

V

verification    
BGP configuration    1
BGP groups    1
BGP peer reachability    1
BGP peers (neighbors)    1
firewall filter handles fragments    1
IGMP version    1
multicast SAP and SDP    1
OSPF host reachability    1
OSPF neighbors    1
OSPF routes    1
OSPF-enabled interfaces    1
PIM mode and interface configuration    1
PIM RP address    1
PIM RPF routing table    1
RIP host reachability    1
RIP message exchange    1
RIP-enabled interfaces    1
stateless firewall filter actions    1
stateless firewall filter flood protection    1
stateless firewall filter operation    1
stateless firewall filters    123
stateless firewall statistics    1
static routes in the routing table    1
virtual link, through the backbone area    1