GLBP TLVs and Priority Values

Gateway Load Balancing Protocol (GLBP) utilizes Type-Length-Values (TLVs) to structure its messages, distinguishing between GLBP message types. For example, hello uses type value 28 and Request/Response uses type value 20. Multiple TLVs can coexist within a single GLBP packet, allowing multiple message types, like Hello and Request/Response, to be communicated simultaneously, as seen below in this Wireshark capture:

Frame 42: 102 bytes on wire (816 bits), 102 bytes captured (816 bits)
Ethernet II, Src: Cisco_00:01:01 (00:07:b4:00:01:01), Dst: IPv4mcast_66 (01:00:5e:00:00:66)
Internet Protocol Version 4, Src: 192.168.0.10, Dst: 224.0.0.102
User Datagram Protocol, Src Port: 3222, Dst Port: 3222
Gateway Load Balancing Protocol
    Version?: 1
    Unknown1: 0
    Group: 1
    Unknown2: 0000
    Owner ID: c2:00:7c:b8:00:00 (c2:00:7c:b8:00:00)
    TLV l=28, t=Hello
    TLV l=20, t=Request/Response?

The priority value observed in GLBP Hello messages is configurable using the glbp 1 priority command, influencing the selection of the Active Virtual Gateway (AVG). Below you can see this priority value within the Hello TLV:

Frame 42: 102 bytes on wire (816 bits), 102 bytes captured (816 bits)
Ethernet II, Src: Cisco_00:01:01 (00:07:b4:00:01:01), Dst: IPv4mcast_66 (01:00:5e:00:00:66)
Internet Protocol Version 4, Src: 192.168.0.10, Dst: 224.0.0.102
User Datagram Protocol, Src Port: 3222, Dst Port: 3222
Gateway Load Balancing Protocol
    Version?: 1
    Unknown1: 0
    Group: 1
    Unknown2: 0000
    Owner ID: c2:00:7c:b8:00:00 (c2:00:7c:b8:00:00)
    TLV l=28, t=Hello
        Type: Hello (1)
        Length: 28
        Unknown1-0: 00
        VG state?: Active (32)
        Unknown1-1: 00
        Priority: 200
        Unknown1-2: 0000
        Helloint: 3000
        Holdint: 10000
        Redirect: 600
        Timeout: 14400
        Unknown1-3: 0000
        Address type: IPv4 (1)
        Address length: 4
        Virtual IPv4: 192.168.0.1
    TLV l=20, t=Request/Response?

Conversely, the priority within the Request/Response TLV is not configurable; it is internally derived by GLBP via an undisclosed algorithm. This dynamic priority contributes to determining which devices become Active Virtual Forwarders (AVFs), factoring in parameters such as device weight and operational status. The following packet capture shows this priority value:

Frame 42: 102 bytes on wire (816 bits), 102 bytes captured (816 bits)
Ethernet II, Src: Cisco_00:01:01 (00:07:b4:00:01:01), Dst: IPv4mcast_66 (01:00:5e:00:00:66)
Internet Protocol Version 4, Src: 192.168.0.10, Dst: 224.0.0.102
User Datagram Protocol, Src Port: 3222, Dst Port: 3222
Gateway Load Balancing Protocol
    Version?: 1
    Unknown1: 0
    Group: 1
    Unknown2: 0000
    Owner ID: c2:00:7c:b8:00:00 (c2:00:7c:b8:00:00)
    TLV l=28, t=Hello
    TLV l=20, t=Request/Response?
        Type: Request/Response? (2)
        Length: 20
        Forwarder?: 1
        VF state?: Active (32)
        Unknown2-1: 00
        Priority: 167
        Weight: 100
        Unknown2-2: 00384002580000
        Virtualmac: Cisco_00:01:01 (00:07:b4:00:01:01)

While Cisco's official documentation provides limited insight into this particular priority value and its internal mechanisms, analysis suggests this value is derived from a combination of runtime conditions and device-specific metrics. The priority value, evidenced in network packet captures, indicates a commonality but lacks explicit description in public resources.

https://networklessons.com/ip-services/glbp-gateway-load-balancing-protocol