JN0-352 Exam Dumps - Enterprise Routing and Switching, Specialist (JNCIS-ENT)

A GRE-encapsulated packet consists of the original passenger packet wrapped inside a GRE header and a new outer IP delivery header, and the combined size of these two additions is what constitutes GRE's per-packet overhead. The base GRE header, as defined in RFC 2784 and used in its minimal, non-keyed, non-checksummed form (the default and most common deployment), is exactly 4 bytes in length, containing the flags/version field and the protocol type field identifying the encapsulated payload's ether type. Layered around that GRE header is a completely new outer IPv4 header, which itself consumes a standard 20 bytes, containing the tunnel source and destination addresses used for delivery across the underlying network. Adding these two components together — 4 bytes for the GRE header plus 20 bytes for the new outer IPv4 header — yields a total of exactly 24 bytes of overhead imposed on every encapsulated packet, which is precisely why Junos automatically sets the default protocol MTU on gr- tunnel interfaces to 1476 bytes (1500 minus 24) to prevent oversized, post-encapsulation packets from requiring fragmentation on a standard 1500-byte Ethernet path. The 20-byte figure alone describes only the new IP header in isolation and omits the GRE header itself, while 12 and 32 bytes do not correspond to any standard, non-keyed GRE encapsulation configuration and are included purely as plausible-looking distractors. Reference topics: Junos Enterprise Routing – Tunneling, GRE Header Overhead and MTU Planning.

By default, EBGP sessions are built with the expectation that peers are directly connected at Layer 3, and Junos enforces this by sending EBGP packets with a TTL of 1. When loopback addresses are used as the session endpoints instead of the directly connected physical interface addresses, the resulting IP packets must traverse at least one additional router hop between the two loopbacks, and a TTL of 1 will expire in transit before reaching the remote loopback, causing the TCP handshake — and therefore the BGP session — to fail even though a valid route to the peer's loopback exists. The multihop statement (configured as an EBGP multihop parameter with an accompanying TTL value, most commonly set to 2 for a single intermediate hop) instructs Junos to increase the TTL used for that peer's BGP packets so they survive the extra hop count between loopbacks. This is a mandatory addition any time an EBGP session is deliberately built against non-directly-connected addresses, whereas IBGP sessions, by contrast, do not have this default restriction because IBGP already assumes peers may be multiple hops apart within the same AS. Multipath affects load-balancing across equal-cost paths, damping suppresses flapping routes, and passive controls whether a router waits for the remote peer to initiate the TCP connection — none of these resolve the TTL expiration problem inherent to loopback-based EBGP peering. Reference topics: Junos Enterprise Routing – BGP, Configuring EBGP Multihop Sessions.

GRE encapsulation adds a fixed 24 bytes of overhead to every packet it carries — 4 bytes for the GRE header itself and 20 bytes for the new outer IPv4 delivery header. When a host transmits a full 1500-byte Ethernet payload into a gr- interface, the resulting encapsulated packet becomes 1524 bytes, which exceeds the physical interface's standard 1500-byte MTU and forces fragmentation or drops if the don't-fragment bit is set. The correct remediation is to reduce the effective MTU seen by end hosts so that encapsulated packets never exceed the physical link's transmission limit. Junos automatically applies this logic to gr- logical interfaces, which default to a 1476-byte protocol MTU (1500 minus 24), but when the underlying physical interface or a manually configured path is involved, administrators must explicitly size the interface MTU to 1476 bytes to prevent post-encapsulation oversize packets. Allowing fragmentation with clear-dont-fragment is a workaround that increases CPU load and can degrade performance rather than solving the root cause, BFD addresses link-failure detection rather than MTU sizing, and ToS copying affects only the type-of-service byte, not packet length. Reference topics: Junos Enterprise Routing – Tunneling, GRE Encapsulation and MTU Considerations; Junos OS Configuring GRE Tunnel Interfaces.