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Configuration Guide - IP Routing 01

NE05E and NE08E V300R003C10SPC500

This is NE05E and NE08E V300R003C10SPC500 Configuration Guide - IP Routing
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Configuring Path MTU Auto Discovery

Configuring Path MTU Auto Discovery

Path MTU auto discovery allows BGP to discover the smallest MTU value on a path so that BGP messages are transmitted based on the path MTU. This function improves transmission efficiency and BGP performance.

Usage Scenario

The link-layer MTUs of different networks through which a path traverses may vary from each other. The smallest MTU on the path is called the path MTU.

Path MTUs vary with the path, and the path MTUs may be different in opposite directions. With path MTU auto discovery, BGP can discover the Path MTU from the source to the destination. The path MTU will be encapsulated into TCP IP data during BGP message transmission.

In Figure 10-9, a BGP peer relationship is set up between Device A and Device D. BGP messages are encapsulated into TCP data packets for transmission. Therefore, Device A sends TCP data packets to Device D based on the default maximum segment size (MSS). As a result, a lot of BGP messages are fragmented into different packets, and the number of ACK messages corresponding to these messages increases, leading to a low transmission efficiency. To resolve this issue, configure Path MTU auto discovery. In Figure 10-9, the path MTU between Device A and Device D is 1496. With path MTU auto discovery, BGP messages are transmitted based on the path MTU (1496), which speeds up BGP message transmission and improves BGP performance.

Figure 10-9 Networking with path MTU auto discovery

Pre-configuration Tasks

Before configuring path MTU auto discovery, configure basic BGP functions.

Procedure

  1. Run system-view

    The system view is displayed.

  2. Run bgp as-number

    The BGP view is displayed.

  3. Run peer { group-name | ipv4-address } path-mtu auto-discovery

    Path MTU auto discovery is enabled.

    By default, path MTU auto discovery is disabled.

    After the command is run, a BGP peer learns the path MTU, preventing BGP messages from being fragmented during transmission.

    NOTE:

    A BGP message from one end to the other may travel a path different from the path used by the ACK message that is responded by the other end. Therefore, running this command on both ends is recommended so that both peers exchange messages based on the path MTU.

  4. Run quit

    Return to the system view.

  5. Run tcp timer pathmtu-age age-time

    The aging time is set for an IPv4 path MTU.

    By default, the IPv4 path MTU aging time is 0 seconds. An IPv4 path MTU does not age.

    The path MTUs vary with the path. If there are multiple routes between two communication hosts and the routes selected for packet transmission change frequently, configure the path MTU aging time so that the system updates path MTUs based on the path MTU aging time, increasing the transmission efficiency.

  6. Run commit

    The configuration is committed.

Checking the Configurations

After configuring path MTU auto discovery, run the following commands to check the previous configuration.

  • Run the display bgp peer [ ipv4-address ] verbose command to check whether path MTU auto discovery has been successfully configured.

# After configuring path MTU auto discovery, view detailed information about the BGP peer at 10.1.1.2.

<HUAWEI> display bgp peer 10.1.1.2 verbose
         BGP Peer is 10.1.1.2,  remote AS 65009
         Type: IBGP link
         BGP version 4, Remote router ID 4.4.4.4
         Update-group ID: 1
         BGP current state: Established, Up for 1d00h25m21s
         BGP current event: KATimerExpired
         BGP last state: OpenConfirm
         BGP Peer Up count: 1
         Received total routes: 0
         Received active routes total: 0
         Advertised total routes: 0
         Port:  Local - 179      Remote - 50450
         Configured: Connect-retry Time: 32 sec
         Configured: Active Hold Time: 180 sec   Keepalive Time:60 sec
         Received  : Active Hold Time: 180 sec
         Negotiated: Active Hold Time: 180 sec   Keepalive Time:60 sec
         Peer optional capabilities:
         Peer supports bgp multi-protocol extension
         Peer supports bgp route refresh capability
         Peer supports bgp 4-byte-as capability
         Address family IPv4 Unicast: advertised and received
 Received: Total 1467 messages
                  Update messages                0
                  Open messages                  1
                  KeepAlive messages             1466
                  Notification messages          0
                  Refresh messages               0
 Sent: Total 1468 messages
                  Update messages                0
                  Open messages                  2
                  KeepAlive messages             1466
                  Notification messages          0
                  Refresh messages               0
 Authentication type configured: None
 Last keepalive received: 2011/04/19 17:13:09 UTC-11728:00
 Minimum route advertisement interval is 15 seconds
 Optional capabilities:
 Route refresh capability has been enabled
 4-byte-as capability has been enabled
 Peer Preferred Value: 0
 Routing policy configured:
 No routing policy is configured
 Path MTU discovery has been configured
NOTE:

The message of Path MTU discovery has been configured will be displayed only after the display bgp peer ipv4-address verbose command is run on the NE where path MTU auto discovery has been enabled.

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Updated: 2019-01-14

Document ID: EDOC1100058916

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