Example: Configuring Error Handling for BGP Update Messages

Requirements

• Configure router interfaces.
• Configure an interior gateway protocol (IGP).
• Configure BGP.
• Configure routing policies.

Overview

When a routing device receives an update message with a malformed attribute, the router is required to reset the session. This is specified in RFC 4271, A Border Gateway Protocol 4 (BGP-4). Session resets impact not only routes with the offending attribute, but also other valid routes exchanged over the session. Moreover, this behavior can present a potential security vulnerability in the case of optional transitive attributes. To minimize the impact on routing made by malformed update messages, the Internet draft draft-ietf-idr-error-handling-01.txt, Revised Error Handling for BGP UPDATE Messages specifies modifications for handling BGP update message with malformed attributes. The new error handling allows for maintaining the established session and keeping the valid routes exchanged, while removing the routes carried in the malformed UPDATE message.

In Figure 1, Device R1 has an internal BGP peering session with Device R0, and an external BGP peering session with Device R2.

Figure 1: BGP Error Handling Example Topology

To protect against malformed update messages causing network instability, Device R1 has BGP error handling configured, as shown here:

By default, a BGP message is considered to be malformed when any one of the message attributes is malformed. When a router participating in a BGP session receives a malformed update message, the entire session is reset. The bgp-error-tolerance statement overrides this behavior so that the following BGP error handling is in effect:

• For fatal errors, Junos OS sends a notification message titled Error Code Update Message and resets the BGP session. An error in the MP_{UN}REACH attribute is considered to be fatal. The presence of multiple MP_{UN}REACH attributes in one BGP update is also considered to be a fatal error. Junos OS resets the BGP session if it cannot parse the NLRI field or the BGP update correctly. Failure to parse the BGP update packet can happen when the attribute length does not match the length of the attribute value.
• For some nonfatal errors, Junos OS treats all the routes contained in the malformed BGP update message as withdrawn routes and installs them as hidden, unless the keep none statement is included in the BGP is configuration. Junos OS uses this error handling approach for the cases that involve any of the following attributes: ORIGIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, LOCAL_PREF, ORIGINATOR, CLUSTER, ATTRSET, PMSI, Community, and Extended Community. In addition, if any of the mandatory well-known path attributes is missing, Junos OS treats the BGP update as malformed. To limit the memory usage of these malformed hidden routes, Junos OS stops installing new malformed hidden routes after the maximum number of such malformed hidden routes is reached. In this example, the maximum number is set to 5, using the malformed-route-limit statement. The default value is 1000. Optionally, you can allow an unlimited number of routes hidden due to malformed attributes. Do this by including the no-malformed-route-limit statement.
• For other nonfatal errors, Junos OS discards the malformed path attributes and continues to process the BGP update message. It is unsafe to use this approach on the path attributes that might affect route selection or installation. Junos OS uses this error handling approach for the cases that involve any of the following attributes: ATOMIC_AGGREGATE, AGGREGATOR, AGGREGATOR4, and AS4PATH.

To facilitate troubleshooting of malformed packets, Junos OS logs the error listing the malformed path attribute code, flag, length, information about the peer and family, and the first prefix from the malformed BGP update. Logging of the malformed packets might slow Junos OS performance if a significant number of malformed packets is received in a short time. To limit the performance impact, Junos OS implements an algorithm to log a malformed update, suppress logging for an interval, and log a summary. When the logging suppression timer expires, the software logs the total number of malformed attributes received during the interval. In this example, the timer is set to 10 seconds, using the malformed-update-log-interval statement. The default value is 300 seconds(5 minutes).

CLI Quick Configuration shows the configuration for all of the devices in Figure 1.

The section Step-by-Step Procedure describes the steps on Device R1.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Step-by-Step Procedure

1. Configure the router interfaces.
   [edit interfaces]user@R1# set fe-1/2/1 unit 0 description to-R2user@R1# set fe-1/2/1 unit 0 family inet address 10.10.10.1/30
   user@R1# set fe-1/2/0 unit 0 description to-R0user@R1# set fe-1/2/0 unit 0 family inet address 172.16.10.6/30
   user@R1# set lo0 unit 0 family inet address 192.168.0.1/32
2. Configure an interior gateway protocol (IGP), such as OSPF or IS-IS.
   [edit protocols ospf area 0.0.0.0]user@R1# set interface fe-1/2/1.0user@R1# set interface fe-1/2/0.0user@R1# set interface lo0.0 passive
3. Configure the autonomous system (AS) number and router ID.
   [edit routing-options]user@R1# set autonomous-system 64510user@R1# set router-id 192.168.0.1
4. Configure the routing policy.
   [edit policy-options policy-statement local-direct]user@R1# set from protocol [local direct]user@R1# set then accept
5. Configure the EBGP session.
   [edit protocols bgp group external-peers]user@R1# set type externaluser@R1# set export local-directuser@R1# set peer-as 64511user@R1# set neighbor 10.10.10.2
6. Configure the IBGP sessions.
   [edit protocols bgp group internal-peers]user@R1# set type internaluser@R1# set local-address 192.168.0.1user@R1# set export local-directuser@R1# set neighbor 192.168.0.3
7. Enable BGP error tolerance.
   [edit protocols bgp]user@R1# set bgp-error-tolerance
8. (Optional) Configure the log interval.
   [edit protocols bgp bgp-error-tolerance]user@R1# set malformed-update-log-interval 10
9. (Optional) Configure a limit for the number of hidden routes to store.
   [edit protocols bgp bgp-error-tolerance]user@R1# set malformed-route-limit 5

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options, commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the devices, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

• Checking the BGP Neighbor Sessions
• Checking Hidden Routes
• Verifying the Source of the Hidden Routes

Checking the BGP Neighbor Sessions

Purpose

Verify that BGP error tolerance is enabled, and display the counters related to malformed path attributes.

Action

Peer: 10.10.10.2+50058 AS 64511 Local: 10.10.10.1+179 AS 64510
  Type: External    State: Established    Flags: <Sync>
  Last State: OpenConfirm   Last Event: RecvKeepAlive
  Last Error: None
  Export: [ local-direct ] 
  Options: <Preference PeerAS Refresh>
  Holdtime: 90 Preference: 170
  Number of flaps: 0
  Malformed attributes    log interval: 10   route limit: 5
    Attribute:           ORIGIN(1) Last Received: 0 Total Received: 3
    Attribute:       LOCAL_PREF(5) Last Received: 0 Total Received: 2
  Peer ID: 192.168.10.2    Local ID: 192.168.10.1      Active Holdtime: 90
  Keepalive Interval: 30         Group index: 0    Peer index: 0   
  BFD: disabled, down
  Local Interface: fe-1/2/1.0                       
  NLRI for restart configured on peer: inet-unicast
  NLRI advertised by peer: inet-unicast
  NLRI for this session: inet-unicast
  Peer supports Refresh capability (2)
  Stale routes from peer are kept for: 300
  Peer does not support Restarter functionality
  NLRI that restart is negotiated for: inet-unicast
  NLRI of received end-of-rib markers: inet-unicast
  NLRI of all end-of-rib markers sent: inet-unicast
  Peer supports 4 byte AS extension (peer-as 64511)
  Peer does not support Addpath
  Table inet.0 Bit: 10000
    RIB State: BGP restart is complete
    Send state: in sync
    Active prefixes:              0
    Received prefixes:            3
    Accepted prefixes:            0
    Suppressed due to damping:    0
    Advertised prefixes:          2
  Last traffic (seconds): Received 25   Sent 17   Checked 73  
  Input messages:  Total 2702   Updates 10      Refreshes 0     Octets 51652
  Output messages: Total 2701   Updates 6       Refreshes 0     Octets 51571
  Output Queue[0]: 0

Peer: 192.168.10.3+179 AS 64510 Local: 192.168.10.1+51127 AS 64510
  Type: Internal    State: Established    Flags: <Sync>
  Last State: OpenConfirm   Last Event: RecvKeepAlive
  Last Error: None
  Export: [ local-direct ] 
  Options: <Preference LocalAddress Refresh>
  Local Address: 192.168.10.1 Holdtime: 90 Preference: 170
  Number of flaps: 0
  Malformed attributes    log interval: 10   route limit: 5
  Peer ID: 192.168.10.3    Local ID: 192.168.10.1      Active Holdtime: 90
  Keepalive Interval: 30         Group index: 1    Peer index: 0   
  BFD: disabled, down
  NLRI for restart configured on peer: inet-unicast
  NLRI advertised by peer: inet-unicast
  NLRI for this session: inet-unicast
  Peer supports Refresh capability (2)
  Stale routes from peer are kept for: 300
  Peer does not support Restarter functionality
  NLRI that restart is negotiated for: inet-unicast
  NLRI of received end-of-rib markers: inet-unicast
  NLRI of all end-of-rib markers sent: inet-unicast
  Peer supports 4 byte AS extension (peer-as 64510)
  Peer does not support Addpath         
  Table inet.0 Bit: 10001
    RIB State: BGP restart is complete
    Send state: in sync
    Active prefixes:              0
    Received prefixes:            3
    Accepted prefixes:            0
    Suppressed due to damping:    0
    Advertised prefixes:          2
  Last traffic (seconds): Received 5    Sent 24   Checked 51  
  Input messages:  Total 417    Updates 3       Refreshes 0     Octets 8006
  Output messages: Total 421    Updates 2       Refreshes 0     Octets 8136
  Output Queue[0]: 0

Meaning

The Malformed attributes field shows that error tolerance is enabled. The log interval and route limit fields display the configured values.

The attribute counters show that on the EBGP connection, several malformed attributes were received from Device R2.

Checking Hidden Routes

Purpose

View information about hidden routes and learn why they are hidden.

Action

inet.0: 42 destinations, 45 routes (36 active, 0 holddown, 6 hidden)
10.0.0.0/32 (1 entry, 0 announced)
         BGP   
                Next hop type: Router
                Address: 0x93d8b0c
                Next-hop reference count: 5
                Source: 10.10.10.2
                Next hop type: Router, Next hop index: 782
                Next hop: via fe-1/2/1.0, selected
                Session Id: 0x1
                State: <Hidden  Ext>
                Local AS:     1 Peer AS:     1
                Age: 5:32       Metric2: 1 
                Validation State: unverified 
                Task: BGP_1.10.10.5.62+56218
                AS path: I (MalformedAttr) 
                Router ID: 192.168.0.2

10.0.0.1/32 (1 entry, 0 announced)
         BGP   
                Next hop type: Router
                Address: 0x93d8b0c
                Next-hop reference count: 5
                Source: 10.10.10.2
                Next hop type: Router, Next hop index: 782
                Next hop: via fe-1/2/1.0, selected
                Session Id: 0x1
                Indirect next hop: 953c000 - INH Session ID: 0x3
                State: <Hidden Int Ext>
                Local AS:     1 Peer AS:     1
                Age: 5:32       Metric2: 1 
                Validation State: unverified 
                Task: BGP_1.10.10.5.62+56218
                AS path: I (MalformedAttr)
                Router ID: 192.168.0.2

Meaning

The malformed hidden routes are marked with MalformedAttr in the AS path field.

You can remove the hidden routes by running the clear bgp neighbor 10.10.10.2 malformed-route command.

Verifying the Source of the Hidden Routes

Purpose

View information about hidden routes and learn why they are hidden.

Action

inet.0: 42 destinations, 45 routes (36 active, 0 holddown, 6 hidden)
  10.0.0.0/32 (1 entry, 0 announced)
     Nexthop: 10.10.10.2
     Localpref: 100
     AS path: I (MalformedAttr)  

  10.0.0.1/32 (1 entry, 0 announced)
     Nexthop: 10.10.10.2
     Localpref: 100
     AS path: I (MalformedAttr)

Meaning

Junos OS displays MalformedAttr in the AS path field in the output of the show route receive-protocol bgp 10.10.10.2 detail hidden command.

You can remove the hidden routes by running the clear bgp neighbor 10.10.10.2 malformed-route command.