OSPF Max-Metric Router LSA Packet Capture

This packet capture contains an OSPF Link State Update with the router LSA of a router that is configured with max-metric router-lsa. That command turns the router into a “stub router”: it stays in the topology, but every link it advertises gets the maximum metric of 65535, so other routers only send transit traffic through it as a last resort.

The only thing that changes in the LSA is the metric on each link. The OSPF stub router lesson explains when you use max-metric, for example during a reload or maintenance. More captures are in the Packet Capture Library.

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Capture overview

Packet walkthrough

The LS Update (packet 1)

Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 76
        Source OSPF Router: 2.2.2.2
        Area ID: 0.0.0.0 (Backbone)
        Checksum: 0x396a [correct]
        Instance ID: Base IPv4 Unicast Instance (0)
        Auth Type: Null (0)
    LS Update Packet
        Number of LSAs: 1
        LSA-type 1 (Router-LSA), len 48
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x22, (DC) Demand Circuits, (E) External Routing
                ..1. .... = (DC) Demand Circuits: Supported
                .... ..1. = (E) External Routing: Capable
            LS Type: Router-LSA (1)
            Link State ID: 2.2.2.2
            Advertising Router: 2.2.2.2
            Sequence Number: 0x80000004
            Checksum: 0xc752
            Length: 48
            Flags: 0x00
            Number of Links: 2
            Type: Transit  ID: 192.168.24.4    Data: 192.168.24.2    Metric: 65535
                Link ID: 192.168.24.4 - IP address of Designated Router
                Link Data: 192.168.24.2
                Link Type: 2 - Connection to a transit network
                Number of Metrics: 0 - TOS
                0 Metric: 65535
            Type: Transit  ID: 192.168.12.2    Data: 192.168.12.2    Metric: 65535
                Link ID: 192.168.12.2 - IP address of Designated Router
                Link Data: 192.168.12.2
                Link Type: 2 - Connection to a transit network
                Number of Metrics: 0 - TOS
                0 Metric: 65535

Here’s what stands out:

• LS Update, one router LSA (type 1): router 2.2.2.2 floods its own LSA after the configuration change. LS Age 1 means it’s brand new.
• Sequence Number 0x80000004: every new version of the LSA gets a higher sequence number. The first one a router creates is 0x80000001, so this router has updated its LSA three times.
• Two transit links: connections to the networks with DRs 192.168.24.4 and 192.168.12.2.
• Metric 65535 on both: 0xFFFF, the highest value that fits in the 16-bit metric field. The normal metric would be the interface cost, for example 10. Other routers still compute paths through this router, but any path with a normal metric wins.
• Flags: no bits set: max-metric doesn’t use a flag of its own. You can only recognize a stub router by its metrics.